DESCRIPTION: The long term goal of this proposal is to understand the structural principles that govern protein-protein interactions which are central to many important cellular processes. The focus is on the leucine zipper, a short coiled-coil motif that is found in many proteins with diverse functions and which mediates the dimerization of two families of eukaryotic transcription factors. This simple motif will be used to investigate the structure, stability and design of coiled coils. The specific projects are as follows. First, the stabilities and X-ray crystal structures of dimeric and trimeric mutants of the GCN4 leucine zipper will be compared to evaluate the determinants of structural stability and uniqueness. Second, the role of packing in coiled-coil proteins will be assessed by studying the solution properties and crystal structures of designed coiled-coils that switch oligomerization states upon binding hydrophobic ligands. Third, the three-dimensional structure of a novel, designed, heterotrimeric coiled- coil will be determined to investigate the roles of packing and electrostatic complementarily in oligomer choice and specificity. Finally, methodology will be developed to produce designed probes for coiled-coil proteins, and the initial focus will be on the tumor suppressor molecule APC, which is mutant in most colorectal tumors.